At the same time and even back to 20,000 BP kayak based seamen existed within this island arc. Thus sea borne mobility was a given. yet we look at the island arc and we find it mostly above sea level the entire length nicely stopping ice from breaking out into the open ocean south. The whole of the interior sea should have been an ice sheet, at least during the winter. what does become abundently clear though is that you could walk the entire arc all the way to Vancouver Island with occasional application of even canoes, let alone kayaks. At worst, a short strip may well have seen ice flowing into the sea, but with the direction of travel already southward, that would have stopped no one.